As in conventional casting of unreinforced metals, it is typically important to cleanse the alloy of inclusions such as dross, which can become entrained through stirring an unprotected melt too vigorously or thru recycling of solidified alloy.
In conventional casting/melting of Al alloys, the usual procedure for removing these solid impurities from the melt has been to inject a gas into the melt and distribute the gas throughout the melt, preferably in the form of small, discrete bubbles (usually by means of an impeller). The gas attaches to and thereby assists the dross inclusions in rising to the surface where they can be removed, typically by skimming.
For many years, the purifying gas of choice was one of the chlorinaceous gases, and most particularly, chlorine. It was thought that these gases were particularly good at removing the non-metallic inclusions present in the melt. Chlorine, of course, is a highly corrosive and toxic gas, as are many of its compounds. Because not all of the chlorine was consumed in the melt, substantial amounts of residual chlorine gas were liberated by this particular processing treatment, thus giving rise to not insignificant containment and disposal problems because of the health, safety and environmental hazards of chlorine gas. Even if all of the chlorine were to react, many of the reaction products, such as the tetrachlorides of titanium and silicon, for example, are also gases under the process conditions and are nearly as hazardous as chlorine gas itself.
Accordingly, halide salts such as NaCl and KCl have been substituted for the gaseous halogens for the fluxing and degassing operation. The salts are stirred into the melt in molten form and collect at the surface after the cation exchange reaction takes place. This substitution only partly alleviated the chlorinaceous gas problem because TiCl.sub.4 and SiCl.sub.4 gases could still be formed using this approach. Also, the molten salt fluxing technique tends to add sodium and potassium to the melt, which are generally undesirable impurities.
Another response to the attendant problems of fluxing and degassing with chlorinaceous gases was to switch to the use of at least relatively inert gases such as argon and/or nitrogen. While this change substantially addressed the health, safety and environmental problems caused by the halogens, some foundrymen complained that these "inert" gases did not work as effectively as, for example, chlorine in removing non-metallic impurities and hydrogen. This complaint was at least partially addressed through the development of improved means (typically improved impeller designs) for dispersing the inert gases into the melt in the form of very fine bubbles and giving all of the melt adequate opportunity to contact the dispersed gas.
One of the subsidiary issues which arose with the development of metal matrix composites, particularly cast reinforced aluminum composites, was, again, concerned with fluxing and degassing of the composite melt. It appears to be the case that the ceramic reinforcements which are employed in many aluminum matrix composite systems are rather easily and readily "de-wetted" by contact with any of the above-described fluxing/degassing agents, including, most unfortunately, the inert gases.
In its most "benign" form, such de-wetting merely reduces the degree of bonding between the reinforcement particle and the aluminum matrix. Still, such a reduction often has a deleterious effect on the properties, particularly the mechanical properties of the cast composite because such composites, unlike, for example, a typical ceramic matrix composite, typically benefit from strong bonding between the reinforcement and the matrix.
A more serious consequence of the reinforcement de-wetting phenomenon resulting from an unmodified application of the fluxing/degassing procedures developed for unreinforced alloys to their composite counterparts is the wholesale loss of the ceramic reinforcement from the melt. It appears to be the case, at least in several reinforced aluminum casting systems, that the ceramic reinforcement prefers to contact the gases which are injected into the melt rather than the aluminum matrix metal. The result of this preference is that ceramic particulates, for example, can become "captured" by gas or salt bubbles and be floated right out of the melt. Clearly, the fluxing and degassing procedures developed for unreinforced alloys could not be applied directly to composite melts. On the other hand, Provencher et al. have harnessed this phenomenon as a means for reclaiming a metal matrix composite material. Specifically, U.S. Pat. No. 5,080,715 teaches injection of a molten salt and a gas into a composite melt and mixing to form salt coated gas bubbles. Upon contact of the wetted reinforcing particles with the salt coated bubbles, the reinforcing particles are de-wetted and caused to rise to the surface along with the salt flux, leaving behind substantially pure metal.
U.S. Pat. No. 4,992,241 to Provencher et al. provides one approach to the problem of fluxing and degassing composite melts without removing the reinforcement particles. The solution provided by Provencher et al. represents a modification of standard fluxing/degassing. Specifically, while maintaining the composite melt at a temperature between 720 and 750.degree. C. and stirring without creating a vortex, a mixture of an inert gas and a reactive gas is injected into the melt near the impeller. The injection and stirring are carefully controlled to disperse the injected gas throughout the melt in the form of fine bubbles. When the dispersion has been achieved (typically after about 10 minutes), the injection is ceased. The composite melt is allowed to rest, still at a temperature of 720-750.degree. C. During this time, hydrogen gas diffuses into the bubbles, which also attach themselves to non-metallic impurities as the bubbles float to the surface to form a dross layer. Also during the rest period, the composite melt is stirred periodically (typically for 5 minutes at 10 minute intervals) to keep the ceramic reinforcement suspended in the melt. The dross layer, containing oxide films and other impurities, may then be skimmed off of the surface of the composite melt in the usual fashion.
The commercial acceptance of metal matrix composite materials, particularly cast metal matrix composites will be enhanced by the existence of procedures for recycling such material. Even if one were to ignore all of the costs associated with disposing of a material once an article fabricated from that material has exhausted its useful life, a strong incentive to recycle castable materials still exists because, even when a perfect casting is made, a considerable amount of ancillary cast material remains, most notably in the form of casting gates and runners.
The use of recycled cast metal matrix composites can introduce potentially deleterious oxide inclusions, most especially oxide "skins" which can become trapped in the remelt. Unless proactive measures are taken to address this issue, casting problems are likely to result. For example, the presence of such oxide skins in the casting may drastically impair the mechanical strength and toughness of the casting. Moreover, attempts to prevent such oxide skins from getting into the casting as by, for example, filtering, run the risk of clogging the filter with oxide skin, thereby impairing further transport of composite material into the casting. Instead, the oxide skins, or "dross inclusions" in general must either be removed or otherwise rendered as harmless as possible before the composite material is recast.